Solid state photomultiplier with improved pulse shape readout

ABSTRACT

Embodiments of a solid state photomultiplier are provided herein. In some embodiments, a solid state photomultiplier may include a plurality of pixels, wherein each pixel of the plurality of pixels comprises a plurality of subpixels; and a first set of buffer amplifiers, wherein each buffer amplifier of the first set of buffer amplifiers is respectively coupled to a subpixel of the plurality of subpixels.

CROSS REFERENCE TO RELATED APPLICATION

This patent application claims the benefit of priority, under 35 U.S.C.§119, of U.S. Provisional Patent Application Ser. No. 62/053,454, filedSep. 22, 2014, titled “SOLID STATE PHOTOMULTIPLIER WITH IMPROVED PULSESHAPE READOUT” the entire disclosure of which is incorporated herein byreference.

BACKGROUND

Solid state photomultipliers (SSPMs), which are also commonly referredto as MicroPixel Photon Counters (MPPC) or MicroPixel AvalanchePhotodiodes (MAPD) have become popular for use as photosensors. Forexample, SSPMs have been employed in scintillator based nucleardetectors. Typically, SSPMs are implemented as Silicon Photomultipliers(SiPM). The Silicon Photomultiplier (SiPM) is a multipixel array ofavalanche photodiodes with a number up to a few thousand independentmicropixels (with typical size of 10-100 microns) joined together oncommon substrate and working on common load. Each pixel detects thephotoelectrons with a gain of about 10⁶.

Conventionally, the output of an SSPM pixel is connected to a front endbuffer amplifier, which can be implemented as a transimpedanceamplifier. Using this conventional arrangement can result in a readoutpulse from the SSPM having a readout pulse shape that exhibits a fastrise time (e.g., <1 ns) and a relatively slow fall time (e.g., 10-50ns). However, the inventors have observed that as the size of the SSPMincreases, the readout pulse shape response degrades significantly dueto increased parasitic capacitance and inductance in combination withintrinsic impedance of each SSPM pixel.

Thus, the inventors have provided an improved solid statephotomultiplier.

BRIEF DESCRIPTION

Embodiments of a solid state photomultiplier are provided herein. Insome embodiments, a solid state photomultiplier may include a pluralityof pixels, wherein each pixel of the plurality of pixels comprises aplurality of subpixels; and a first set of buffer amplifiers, whereineach buffer amplifier of the first set of buffer amplifiers isrespectively coupled to a subpixel of the plurality of subpixels.

In some embodiments, a silicon photomultiplier array may include aplurality of subpixels arranged in groups to form a pixel; a pluralityof buffer amplifiers respectively coupled to the plurality of subpixels;and a plurality of secondary buffer amplifiers, wherein each group ofsubpixels is coupled to a secondary buffer amplifier of the plurality ofsecondary buffer amplifiers.

In some embodiments, a method for monitoring a solid statephotomultiplier may include monitoring a parameter of a plurality ofsubpixels of a solid state photomultiplier, wherein the plurality ofsubpixels are arranged in groups to form a pixel, and wherein eachsubpixel has a buffer amplifier coupled thereto; determining whether adisablement of a subpixel of the plurality of subpixels or an adjustmentof at least one of a V_(bias) or gain of the buffer amplifier of thesubpixel is needed; and providing a signal to the buffer amplifier todisable the subpixel or adjust at least one of the V_(bias) or gain ofthe buffer amplifier.

The foregoing and other features of embodiments of the present inventionwill be further understood with reference to the drawings and detaileddescription.

DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 illustrates a portion of an exemplary solid state photomultiplier(SSPM) array in accordance with some embodiments of the presentinvention.

FIG. 2 illustrates a block diagram of an exemplary embodiment of anSSPM-based detector in accordance with some embodiments of the presentinvention.

FIG. 3 illustrates a portion of an exemplary SSPM in accordance withsome embodiments of the present invention.

FIG. 4 illustrates a partial electrical schematic of the portion of theSSPM illustrated in FIG. 3.

FIG. 5 illustrates a portion of an exemplary SSPM in accordance withsome embodiments of the present invention.

FIG. 6 illustrates a partial electrical schematic of the portion of theSSPM illustrated in FIG. 5.

FIG. 7 illustrates a portion of an exemplary SSPM in accordance withsome embodiments of the present invention.

FIG. 8 is a flow diagram depicting an adjustment of a voltage and/orbias of a buffer amplifier in accordance with some aspects of thepresent invention.

FIG. 9 is a graphical depiction of a first temperature curve (T1) and asecond temperature curve (T2) of gain as a function V_(bias).

FIG. 10 illustrates an exemplary feedback loop for a portion of a SSPMin accordance with some embodiments of the present invention.

Unless otherwise indicated, the drawings provided herein are meant toillustrate features of embodiments of the disclosure. These features arebelieved to be applicable in a wide variety of systems comprising one ormore embodiments of the disclosure. As such, the drawings are not meantto include all conventional features known by those of ordinary skill inthe art to be required for the practice of the embodiments disclosedherein.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention are directed to improvingfunctionality of a solid state photomultiplier (SSPM). In someembodiments, the inventive SSPM may include one or more bufferamplifiers at subpixel levels. Moreover, the buffer amplifiers may bemultiplexed, thereby providing the above benefits without increasing anumber of readout electronics or complexity of the system. In someembodiments, the buffer amplifiers may be monitored and/or adjusted tocompensate for temperature and process nonuniformity or disabled to turnoff failed or malfunctioning subpixels.

FIG. 1 illustrates a portion of an exemplary SSPM array 110 (e.g., anSiPM) in accordance with some embodiments of the present invention. Thearray 110 can include pixel areas 112 and each pixel area 112 caninclude an SSPM (pixel 114). Each pixel 114 can be formed of an array ofmicrocells 116. The microcells 116 that form the pixels 114 can beimplemented as a two dimensional array having a specified dimension,e.g., from about 10 to about 100 microns, and a specified spatialdensity, e.g., about 100 to about 10,000/sq. mm. In some embodiments,the SSPM array 110 can be incorporated into a high energy detector, suchas a scintillator-based detector or can be used for detecting singlephotons or any other light pulses (multiple photons).

FIG. 2 illustrates an exemplary embodiment of a detector including oneor more of the pixels 114 of FIG. 1. The detector can be implemented ina nuclear detector (e.g., X-ray imaging system) and/or an opticaldetector (e.g., a light detector). Each microcell 116 of the pixel canbe formed by an avalanche photodiode (APD) 218 operating in Geiger modeand a quenching element 220. In exemplary embodiments, the APDs 218 ofthe microcells 116 can be formed using one or more semiconductormaterials, such as Silicon (Si), Silicon Carbide (SiC), Germanium (Ge),Indium Gallium Arsenide (InGaAs), Gallium nitride, Mercury CadmiumTelluride (HgCdTe), and/or any other suitable material(s). In oneembodiment, the array of microcells 116 can be formed on a singlesemiconductor substrate to form the pixel 114.

Each APD 218 in the microcells 116 can have a breakdown voltage (V_(br))of, for example, about 20 to about 2000 Volts and a bias voltage 224 canbe applied to the microcells 116 to configure the APDs 218 in a reversebias mode having an over voltage (V_(ov)) (i.e., the difference betweenthe bias voltage V_(bias) and the breakdown voltage V_(br)). The reversebiased APDs 218 can have an internal current gain of about 100 to about1000 resulting from an avalanche effect within the APDs at bias voltagebelow breakdown. When they operate in Geiger mode, the gain of eachmicrocell 116 is proportional to the over voltage and capacitance ofmicro-cell.

The quenching element 220 in each microcell 116 can be disposed inseries between the bias voltage and the APD 218 or between the APD 218and a common readout bus 232 and can operate to ensure that the APD 218transitions to the quiescent state after a photon is detected. Inexemplary embodiments, the quenching element can be a resistor,transistor, current controlled source, and/or any suitable device ordevices for transitioning the APD 218 to the quiescent state after theAPD 218 detects of a photon. The microcells 116 are connected to eachother in parallel and share a common bias voltage and a common readoutterminal. The output of each microcell 116 is used to generate an output222 of the pixel 114, which can be processed by readout electronics 230.

The output of the microcells 116 can output from the pixel 114 andprocessed via a buffer amplifier 226. The output 222 of the pixel 114can take the form of one or more electrical pulses (“readout pulses”).The readout pulses can have an associated discharge time for which amagnitude of the readout pulse increases and an associated recharge timefor which the magnitude of the readout pulse decreases.

A rate at which the magnitude decreases during the recharge time (i.e.,a recharge rate) can generally be determined by a capacitance associatedwith the APDs 218 of the SSPM and the impedance of the quenchingelements 220. For example, when the quenching elements are resistors,the rate can be defined by the RC time constant formed by thecapacitance of the APDs 218 and the resistance of the quenchingresistors. In conventional readout configurations of SSPMs. the timeconstant can cause the recharge portion of the readout pulse to have along tail (e.g., about 10-50 ns).

In some embodiments, a frequency dependent input impedance circuit 228can be disposed between the output 222 of the pixel 114 and the input ofthe buffer amplifier 226 to provide a frequency dependent impedance. Insome embodiments, the frequency dependent input impedance circuit 228can be part of buffer amplifier 226. When present, the input impedancecircuit 228 can be configured to shape the recharge portion of thereadout pulse. For example, the input impedance circuit 228 can be usedto control a voltage received at the input of the buffer amplifier 226to minimize the amplification of the recharge portion of a readout pulsefrom the pixel 114.

As discussed above, the buffer amplifier 226 receives output from thepixel 114. In some embodiments, the buffer amplifier can be implementedas a transimpedance amplifier. The buffer amplifier can output theamplified signal to readout electronics 230 downstream of the bufferamplifier 226 for further processing by the readout electronics 230,which can include amplifiers, analog-to-digital converters, and/or anyother suitable electronics.

In conventional SSPM/pixel 114 configurations, output of the microcells116 can output from the pixel 114 to the buffer amplifier 226 in asingle cumulative signal (e.g., such as described above with respect toFIGS. 1 and 2). However, the inventors have observed that, as the sizeof the pixel 114 increases, the resultant readout pulse provided to thereadout electronics degrades. While not intending to be bound by theory,the inventors believe that such degradation may be caused by anincreased parasitic capacitance and inductance in combination with anintrinsic impedance of each pixel 114 and associated packaging.

As such, in some embodiments, each pixel 114 may be further divided intosubpixels 302, wherein each subpixel 302 is coupled to a respectivebuffer amplifier 304, for example, such as shown in FIG. 3. As usedherein, coupling of the subpixel 302 and buffer amplifier 304 mayinclude any known coupling mechanism known in the art, for example acoupling via separate conductive element or integration of the bufferamplifier 304 into the subpixel 302 during the fabrication of thesubpixel 302.

The pixel 114 may be divided into any number of subpixels 302 suitableto facilitate the improved pulse shape response of the pixel 114. Forexample, referring to the partial view of a single pixel 114 in FIG. 3,in some embodiments, each pixel 114 may comprise four or more subpixels302 each having a respective buffer amplifier 304 coupled thereto.Referring to FIG. 4, in such embodiments, the buffer amplifiers 304 maybe coupled to one another in parallel and having a single output 402 toprovide the processed signal to, for example, one or more othercomponents of the array (e.g., the, readout electronics 230, impedancecircuit 228, array level buffer amplifier 226, or the like). Couplingthe buffer amplifiers 304 in such a manner allows for the inclusion ofthe buffer amplifiers 304 without having to increase a number of readoutelectronics channels and system complexity.

In some embodiments, the buffer amplifiers may be multiplexed or groupedtogether via one or more secondary or tertiary buffer amplifiers. Forexample, referring to FIG. 5, in some embodiments, a group 502 of bufferamplifiers 304 may be coupled to a secondary buffer amplifier 504. Thebuffer amplifiers 304 may be grouped in any manner suitable tofacilitate improving the readout pulse shape of the array. For example,each group 502 of buffer amplifiers 304 may include buffer amplifiers304 from one subpixel 302 or more than one subpixel 302.

Referring to FIG. 6, in some embodiments, the secondary bufferamplifiers 504 may be coupled to one another in parallel having a singleoutput 602 to provide the processed signal to, for example, one or moreother components of the array (e.g., the, readout electronics 230,impedance circuit 228, array level buffer amplifier 226, or the like).

Although, only two levels of buffer amplifiers (buffer amplifiers 304and secondary buffer amplifiers 504) are shown in FIG. 6, it is to beunderstood that any number of levels may be utilized to facilitateimproving the readout pulse shape of the array. For example, in someembodiments, the secondary buffer amplifiers 504 may be grouped in amanner similar to the buffer amplifiers 304 and coupled to a tertiarybuffer amplifier (shown in phantom at 604) or tertiary set of bufferamplifiers. Referring to FIG. 7, in such embodiments, the bufferamplifiers (e.g., buffer amplifiers 304, secondary buffer amplifiers504, tertiary buffer amplifiers 604, or the like) may be grouped in anymanner suitable to facilitate improving the readout pulse shape of thearray. For example, in some embodiments, each group 502 may comprise aplurality of buffer amplifiers 304 (e.g., more than 1, such as 2, 4 orthe like) coupled to a secondary buffer amplifier 504, wherein aplurality of secondary buffer amplifiers 504 (e.g., more than 1, such as2, 4 or the like) may be coupled to a tertiary buffer amplifier 604,such as shown in the figure.

In any of the above embodiments, the buffer amplifiers (e.g., bufferamplifiers 304, secondary buffer amplifiers 504 or tertiary bufferamplifiers 604) may be fabricated via any process known in the art. Forexample, in some embodiments the buffer amplifiers will be producedduring one or of the semiconductor fabrication processes (e.g, CMOS,MOSFET, or the like) typically utilized to fabricate one or morecomponents of the SSPM array. In such embodiments, the desired placementand coupling of each of the buffer amplifiers may be accomplishedthrough various features formed in one or more layers of the structure.In addition, such fabrication techniques may facilitate the integrationof the buffer amplifiers into the SSPM at subpixel, pixel or arraylevel.

The inventors have observed that due to process and temperaturevariation, the gain of each of the buffer amplifiers (e.g., bufferamplifiers 304, 504, 604 described above) and/or the breakdown voltage(V_(br)) of the SSPM array 110 may vary, thereby introducing gain andsignal response non-uniformities across the pixels and degradation ofthe pulse shape readout. As such, in some embodiments, one or moreparameters of each of SSPM subpixel and the buffer amplifiers may bemonitored and/or adjusted to provide a substantially uniform gain andsignal response between SSPM subpixels and the buffer amplifiers.

For example, in some embodiments, a substantially uniform gain betweenthe SSPM (e.g., pixel 114 described above)/SPAD (e.g., breakdown voltage(V_(br))) and the buffer amplifiers may be desirable to facilitate animproved signal response uniformity. In such embodiments, gainadjustments may be facilitated either by varying anode voltages providedby the buffer amplifiers or direct adjustment of the gains of the bufferamplifiers. Such adjustments may be accomplished by any suitablemechanism known in the art. In some embodiments, the adjustments may beperformed as a function of an integrated feedback loop (e.g., utilizingfeedback circuitry) thus providing an automated system for providinguniformity between the SPPM and buffer amplifiers. In any of the aboveembodiments, after the gain for each buffer and sub pixel arecalibrated, the gain may be maintained and local temperature changes maybe monitored and compensated using components with a substantiallysimilar temperature coefficient (TempCo) as V_(br) in the feedbackcircuitry.

The inventors have further observed that variations in temperature ofthe pixel may cause a malfunction or degradation of the signal providedby the pixel. As such, in some embodiments, the V_(bias) and/or gain maybe adjusted to compensate for temperature changes of the subpixels. Thetemperature may be sensed via any mechanism suitable to accuratelydetect the temperature (e.g., sensor described below with respect toFIG. 10). Once the temperature information is extracted and converted toelectrical signal, a feedback control circuit may automatically adjustthe gain of the buffer amplifier to compensate the effects caused by thescintillator and V_(br) variation due to temperature change. Forexample, referring to the graphical depiction of a first temperaturecurve (T1) and a second temperature curve (T2) of the gain as a functionV_(bias), as shown in FIG. 9, a shift from T1 to T2 may be facilitated,or compensated for, by adjusting the V_(bias) (e.g., from V1 to V2)while maintaining a constant gain (e.g., G1) or adjusting the gain ofamplifier (e.g., from G1 to G2) while maintaining a constant V_(bias)(e.g., V1).

In some embodiments, the monitoring of the temperature and adjustment ofthe V_(bias) and/or gain may be continuous, for example, such as part ofa feedback loop. For example, referring to FIG. 10, in some embodiments,a temperature of the pixel 114 may be continuously monitored via asensor 1002 which in turn provides feedback to the buffer amplifier 304to facilitate adjustments in the gain or V_(bias) of the bufferamplifier 304, for example, such as discussed above.

Referring to the exemplary process flow for monitoring a SSPM 110 inFIG. 8, in some embodiments, one or more parameters of the pixel 114 orsubpixel (e.g., subpixel 302 as described above) may be monitored (shownat 802). The one or more parameters may include any parameter indicativeof operation of the SSPM, for example such as the parameters describedabove (e.g., temperature, V_(bias), gain or the like). Next, at 804 adetermination is made as to whether an adjustment of the V_(bias) and/orgain of the pixel 114 is needed. If no such adjustment is needed the oneor more parameters may be continuously monitored at 802. If anadjustment is needed, the magnitude of the adjustment is determined at806 and provided to control circuitry 810. The control circuitry 810then processes the information related to the adjustment and provides asignal 808 that is indicative of such an adjustment. Based on the signal808, the gain or V_(bias) of the buffer amplifier 304,504,226/604 isadjusted.

In some embodiments, the above described process flow may be continuous,for example, such as part of a feedback loop. Although shown as aseparate component, it is to be understood that the control circuitry810 may be integrated into the array at any level, for example, such asthe pixel level, subpixel level, or the like.

Thus, an improved solid state photomultiplier has been provided herein.In at least some embodiments, the inventive SSPM may include one or morebuffer amplifiers at subpixel levels that may advantageously improve thepulse shape readout of the SSPM as compared to conventionally configuredSSPMs. In addition, in at least some embodiments, the buffer amplifiersmay be monitored and/or adjusted to compensate for temperature andprocess nonuniformity.

Ranges disclosed herein are inclusive and combinable (e.g., ranges of“about 10-50 ns”, is inclusive of the endpoints and all intermediatevalues of the ranges of “about 10-50 ns”, etc.). “Combination” isinclusive of blends, mixtures, alloys, reaction products, and the like.Furthermore, the terms “first,” “second,” and the like, herein do notdenote any order, quantity, or importance, but rather are used todistinguish one element from another, and the terms “a” and “an” hereindo not denote a limitation of quantity, but rather denote the presenceof at least one of the referenced item. The modifier “about” used inconnection with a quantity is inclusive of the state value and has themeaning dictated by context, (e.g., includes the degree of errorassociated with measurement of the particular quantity). The suffix“(s)” as used herein is intended to include both the singular and theplural of the term that it modifies, thereby including one or more ofthat term (e.g., the colorant(s) includes one or more colorants).Reference throughout the specification to “one embodiment”, “someembodiments”, “another embodiment”, “an embodiment”, and so forth, meansthat a particular element (e.g., feature, structure, and/orcharacteristic) described in connection with the embodiment is includedin at least one embodiment described herein, and may or may not bepresent in other embodiments. In addition, it is to be understood thatthe described elements may be combined in any suitable manner in thevarious embodiments.

In describing exemplary embodiments, specific terminology is used forthe sake of clarity. For purposes of description, each specific term isintended to at least include all technical and functional equivalentsthat operate in a similar manner to accomplish a similar purpose.Additionally, in some instances where a particular exemplary embodimentincludes a plurality of system elements, device components or methodsteps, those elements, components or steps may be replaced with a singleelement, component or step. Likewise, a single element, component orstep may be replaced with a plurality of elements, components or stepsthat serve the same purpose. Moreover, while exemplary embodiments havebeen shown and described with references to particular embodimentsthereof, those of ordinary skill in the art will understand that varioussubstitutions and alterations in form and detail may be made thereinwithout departing from the scope of the invention. Further still, otheraspects, functions and advantages are also within the scope of theinvention.

1. A solid state photomultiplier, comprising: a plurality of pixels,wherein each pixel of the plurality of pixels comprises a plurality ofsubpixels; and a first set of buffer amplifiers, wherein each bufferamplifier of the first set of buffer amplifiers is respectively coupledto a subpixel of the plurality of subpixels.
 2. The solid statephotomultiplier of claim 1, wherein the buffer amplifiers are integrallyformed with the subpixels.
 3. The solid state photomultiplier of claim1, wherein each of the buffer amplifiers are coupled to one another inparallel to form a single output.
 4. The solid state photomultiplier ofclaim 1, wherein the first set of buffer amplifiers comprise a pluralityof groups of buffer amplifiers, wherein each group of buffer amplifiersare coupled to a respective secondary buffer amplifier of a plurality ofsecondary buffer amplifiers.
 5. The solid state photomultiplier of claim4, wherein the plurality of secondary buffer amplifiers are coupled to atertiary buffer amplifier.
 6. The solid state photomultiplier of claim5, wherein the tertiary buffer amplifier is coupled to readoutelectronics.
 7. The solid state photomultiplier of claim 4, wherein theplurality of secondary buffer amplifiers are coupled to readoutelectronics.
 8. A silicon photomultiplier array, comprising: a pluralityof subpixels arranged in groups to form a pixel; a plurality of bufferamplifiers respectively coupled to the plurality of subpixels; and aplurality of secondary buffer amplifiers, wherein each group ofsubpixels is coupled to a secondary buffer amplifier of the plurality ofsecondary buffer amplifiers.
 9. The silicon photomultiplier array ofclaim 8, wherein the buffer amplifiers are integrally formed with thesubpixels.
 10. The silicon photomultiplier array of claim 8, whereineach buffer amplifier of the plurality of buffer amplifiers are coupledto one another in parallel to form a single output.
 11. The siliconphotomultiplier array of claim 8, wherein the plurality of secondarybuffer amplifiers are coupled to a tertiary buffer amplifier.
 12. Thesilicon photomultiplier array of claim 8, wherein the plurality ofsecondary buffer amplifiers are coupled to readout electronics.
 13. Thesilicon photomultiplier array of claim 8, wherein the tertiary bufferamplifier is coupled to readout electronics.
 14. A method for monitoringa solid state photomultiplier, comprising: monitoring a parameter of aplurality of subpixels of a solid state photomultiplier, wherein theplurality of subpixels are arranged in groups to form a pixel, andwherein each subpixel has a buffer amplifier coupled thereto;determining whether an adjustment of at least one of a V_(bias) or gainof the buffer amplifier of the subpixel is needed; and providing asignal to the buffer amplifier to adjust at least one of the V_(bias) orgain of the buffer amplifier.
 15. The method of claim 14, wherein theparameter comprises at least one of a temperature, V_(bias) or gain ofthe subpixel.
 16. The method of claim 14, wherein adjusting at least oneof the V_(bias) or gain of the buffer amplifier comprises adjusting thegain of the buffer amplifier such that the buffer amplifiers of theplurality of subpixels have a substantially uniform gain.
 17. The methodof claim 14, wherein adjusting at least one of the V_(bias) or gain ofthe buffer amplifier comprises adjusting the V_(bias) at the input ofthe buffer amplifier while maintaining the gain of the SPPM subpixelwith buffer amplifier at a constant.
 18. The method of claim 14, whereinadjusting at least one of the V_(bias) or gain of the buffer amplifiercomprises adjusting the gain of the buffer amplifier while maintainingthe V_(bias) at a constant.